This invention relates to a multi-layer coating formed on a glass substrate in such a fashion that the coating is electrically conductive. The glass sheet carrying the electrically conductive coating is then heat strengthened in a conventional glass heating furnace. More particularly, this invention relates to creating a multi-layer electrically conductive coating such that when heat strengthened, the variation in the sheet resistance of the coating is minimized.
Coatings can be formed from a wide variety of materials which are deposited on glass and other materials as substrates, to accomplish a variety of functions. One example of such coated glasses involves the deposition of metal oxides on such substrates, and by the addition of various so-called dopant materials, causing one or more of such metal oxide layers to be electrically conductive.
Such electrically conductive coatings on glass substrates have a wide variety of end uses, some of which require further processing of the coated glass sheets. One such additional processing step is the heat strengthening of the coated glass sheets, which involves a limited amount of controlled reheating and cooling of the coated glass sheet. Thus, heat strengthened glass is more resistant to thermal breakage due to improved surface compressive stress. Generally, glass can be said to be heat strengthened when surface compressive stress is greater than 3,500 pounds per square inch (psi).
Further, heat strengthening of the coated glass, where surface compressive stress is greater than 8,000 psi, is also known as tempering. Tempered glass, upon sufficient impact, will break in a controlled fashion, i.e. into small granular fragments, so as to reduce the likelihood of injury to people or animals who may contact it. Therefore, tempering adds a safety factor where coated glass sheets are used in applications where people may come in contact with them, for example, commercial freezers and refrigerators, such as are used in supermarkets, and/or vehicle windows and the like. Electrically conductive coated glass sheets can be advantageously used in such applications if, by the application of electrical current, the temperature of the glass sheet is raised, thereby removing frost or other moisture which can condense on the glass, thus decreasing the visibility of products which are displayed behind the glass sheets or obscuring the view of the driver of a vehicle in the case of vehicle window.
U.S. Pat. No. 4,547,400 discloses the formation of a coated soda-lime silica glass wherein a first xe2x80x9cseal coatingxe2x80x9d is formed by the deposition of an organic/metallic ion-containing, chlorine-free compound. The stated purpose of the seal coating is to minimize penetration of said coating by chlorine containing materials which would be a potential point of reaction with sodium on the surface of the glass to form crystals of sodium chloride. The formation of sodium chloride is considered to be detrimental as it creates voids in the film leading to light scattering defects and the optically undesirable condition known as xe2x80x9chazexe2x80x9d. A coating of fluorine doped tin oxide is subsequently applied next to the metal oxide seal coating through the decomposition of butyltin trichloride.
U.S. Pat. No. 4,548,836 discloses subject matter similar to the ""400 patent. However, it includes additional information regarding the columnar microstructure of the tin oxide coatings. This patent, however, still requires the use of a chlorine-free compound to form the xe2x80x9cseal coating.xe2x80x9d
U.S. Pat. No. 4,788,079 discloses a method of making haze-free tin oxide coatings. The thrust of the invention of the ""079 patent is the use of monophenyltin trichloride to form a haze-free tin oxide undercoat on glass, on which haze-free conductive doped tin oxide coatings may be formed under process conditions which ordinarily would give hazy coatings, if deposited directly on glass. The ""079 patent is primarily directed to use of a particular organotin compound specifically monophenyltin trichloride.
U.S. Pat. No. 4,859,499 is directed to a method for coating a substrate with an electrically conductive layer. The invention of the ""499 patent concerns the preparation of a metal based powder in a method for pyrolytically depositing this powder upon a substrate in order to form a transparent coating with low emissivity and greater electrical conductivity. This case, however, requires the use of dry indium formate as the precursor material to form the coating of indium oxide upon the surface of the substrate.
It is desirable to be able to control the electrical sheet conductance or its inverse, known as electrical sheet resistance, in electrically conductive coated glass sheets to accommodate the line voltage of the electrical current as applied to the coated glass sheet. For coated glass sheets which have not been heat strengthened, the conductivity or sheet resistance can be controlled with a reasonable degree of precision. It has been discovered, however, that upon heat strengthening, the sheet resistance of the electrically conductive coating changes, sometimes to a significant degree, and not in a predictable fashion. Therefore, it would be desirable to develop a glass coating which can be heat strengthened while still maintaining the sheet resistance within an acceptable range of variability.
In accordance with the present invention, a glass substrate is provided which carries a coating of substantially undoped metal oxide deposited on the glass substrate, and a coating of a doped metal oxide deposited directly on the undoped metal oxide coating, wherein the source metal of the undoped and doped metal oxides is the same.
It is an object of the present invention to provide a multi-layer metal oxide coated glass article which, upon heat strengthening, will exhibit a relatively predictable sheet resistance.
It is known that when depositing metal oxide films on glass substrates and when such films obtain a critical thickness, a mottled, reflected color, or iridescence, can occur. It would be desirable to have a heat strengthened metal oxide coated film which was color-neutral by depositing additional coating(s) to suppress such iridescence.
Most glass substrates which would be suitable for the glass sheets to be coated in the present invention are manufactured by the float glass process. While such coatings can be deposited in several ways, it would be desirable to form the coatings on-line during the float glass manufacturing process to avoid the cost associated with off-line coating operations. An additional advantage of the on-line chemical vapor deposition process is that the coatings formed, termed pyrolytic coatings, are very durable and do not require special handling, such as may be necessary for off-line sputter coated products. Accordingly, it would be desirable to form the multi-layer electrically conductive coatings with stable sheet resistance by the chemical vapor deposition process in the environment of the float bath.